This invention relates to a velocity error compensator that is adapted for use in a time base corrector and, more particularly, to an improved velocity error compensator which functions to more accurately compensate for actual velocity errors that may be present in the information signals which are supplied to the time base corrector.
Time base correctors are particularly useful in correcting time base, or frequency and/or phase errors which may be introduced into periodic information signals that are reproduced by a signal playback device. One advantageous application of time base correctors is to correct for such errors in video signals that are reproduced by a video tape recorder (VTR). Although there are, of course, other applications of time base correctors, in the interest of simplification, the following description is limited primarily to the use of such time base correctors in conjunction with a VTR.
Time base errors may be present in the video signals reproduced from a VTR when, for example, the record medium, such as magnetic tape, expands or contracts after the video signals have been recorded thereon. Time base errors also may be present if the speed at which the record medium is transported during a playback operation differs from its speed during the recording operation. Still further, if the rotary speed at which the head (or heads) scans the record medium during the signal playback operation differs from its rotary speed during the signal recording operation, time base errors may be introduced. These time base errors are perceived, in the video picture ultimately reproduced from the played back video signals, as jitter distortion, brightness distortion, improper color display, and the like.
Such time base errors are corrected in time base correcting apparatus, one example of which is described in U.S. Pat. No. 3,860,952. In a typical time base corrector, the reproduced video signals are converted from analog form into digital form and are temporarily stored in a main memory. The digitized video signals are written into the main memory at a write-in clock rate which varies in accordance with detected time base errors. For example, the write-in clock generator may be synchronized to the reproduced horizontal synchronizing signal and/or to the chrominance subcarrier represented by the burst signal included in a composite color video signal. Although the digitized video signals thus are synchronously written into the main memory, they are read out from that memory at a standard, or reference read-out rate. Thus, the digitized video signals read out from the main memory are substantially free of significant time base errors. The read out digitized video signals then are reconverted back into analog form.
Although significant, slowly varying time base errors are corrected by time base correctors of the foregoing type, velocity errors which may be present in the reproduced video signals generally are not taken into account. The expression "velocity error" generally refers to the error in the phase or frequency of the write-in clock with respect to the reproduced video signals over a horizontal line interval. This error may occur because the write-in clock is synchronized only at the beginning of each line interval. Synchronism generally is attained by means of a phase-locked loop that is phase-locked to the horizontal synchronizing pulse and/or to the burst signal, both of which are present only at the beginning of the line interval. Thereafter, the write-in clock remains fixed throughout the remainder of the line interval, even though the frequency and/or phase of the reproduced video signals may vary. Of course, at the beginning of the next-following line interval, the write-in clock is brought into proper synchronism with the video signals. The amount of frequency and/or phase adjustment which is needed at the beginning of the next-following line interval in order to bring the write-in clock into proper synchronism is representative of the size, or quantity, of the velocity error that was present over the just-received line interval. This velocity error, if not corrected, will be perceived as jitter in the video picture which ultimately is reproduced.
One technique for correcting velocity errors is described in U.S. Pat. No. 4,054,903, assigned to the assignee of the present invention. In this patent, velocity errors are detected by sensing the difference between the phase of the burst signal which is reproduced in two successive lines. Such detected velocity errors are represented as voltages, and the velocity error voltages associated with corresponding line intervals are stored in suitable storage devices, such as capacitors. When an individual line interval is read out from the main memory of the time base corrector, its associated velocity error voltage also is read out. This error voltage then is used to modulate the rate at which this line of video signals is read out from the main memory. Velocity error correction thus proceeds on the assumption that the velocity error which may be present in a line interval varies approximately in a uniform, or linear manner over the entire line interval. Thus, the velocity error voltage is integrated to produce a sawtooth-shaped velocity error correction signal. The phase of the read-out clock pulses is modulated with this velocity error correction signal so as to increase (or decrease) uniformly over the line interval, thereby providing an approximate correction for the velocity error.
Another technique for correcting velocity errors is described in U.S. Pat. No. 4,165,524, also assigned to the assignee of the present invention. In this patent, it is assumed that the velocity error which may be present over a line interval is not a linear function. Here, the non-linear velocity error over a particular line interval N is approximated by generating a velocity error correction signal that varies linearly with a first slope during a first quarter of the line interval, then varies linearly with a second, different slope during the next two quarters of the line interval, and then varies with a linear, still different slope over the last quarter of the line interval. The first slope is produced as a function of the velocity error associated with the Nth line interval, in combination with the velocity error of the (N-1)th line interval; the second slope is a function of the velocity error associated solely with Nth line interval; and the third slope is a function of the velocity error of the Nth line interval, in combination with the velocity error associated with the (N+1)th line interval. Although the velocity errors associated with three successive line intervals are used, in combination, to produce the resultant velocity error correction signal, a relatively complicated timing scheme is relied upon in order to read out the different velocity error signals at different times during the Nth line interval. Furthermore, while one velocity error voltage is read out from the storage device, the preceding velocity error voltage must be stored in a sample-and-hold circuit so that it can be combined therewith. Also, the velocity error associated with the Nth line interval must be read out after three-fourths of the (N-1)th line interval has been read out from the main memory.
It is desirable to approximate the non-linear velocity error which may be present in a line interval of video signals with a relatively simpler velocity error compensating circuit that does not require a complicated timing scheme and which obviates the need for additional sample-and-hold circuitry.